Authorization Bypass Through User-Controlled Key Affecting mcp package, versions <0.23.0
Threat Intelligence
Snyk has a proof-of-concept or detailed explanation of how to exploit this vulnerability.
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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Start learning- Snyk IDSNYK-RUBY-MCP-18017728
- published19 Jul 2026
- disclosed17 Jul 2026
- creditSrikanth Ramu
Introduced: 17 Jul 2026
NewCVE-2026-67431 (opens in a new tab)How to fix?
Upgrade mcp to version 0.23.0 or higher.
Overview
mcp is a The official Ruby SDK for Model Context Protocol servers and clients
Affected versions of this package are vulnerable to Authorization Bypass Through User-Controlled Key in lib/mcp/server/transports/streamable_http_transport.rb, which processes tool execution requests to /messages/{session_id} without verifying that the requester owns the identified session. An attacker who obtains a valid session ID can execute unauthorized tool calls and modify state on a victim's session by sending POST requests to /messages/{session_id}, with the tool responses delivered to the victim's SSE stream. Exploitation requires the attacker to obtain a valid session ID out-of-band, such as through network observation or logs.